Interpretive Summary: Body iron stores can be estimated from plasma ferritin concentrations or from the ratio of plasma transferrin receptors to ferritin (TfR/ferritin). Estimates comparing these two methods were calculated using data from four randomized intervention trials conducted by the authors (n=1189 after individuals with infections were excluded). There was a very high correlation between estimates of body iron stores obtained by the two methods. In general ferritin, a cheaper measure, was found to be of similar value to the ratio for assessing iron stores of populations and change in stores after iron interventions. However, at least one indicator of inflammation should also be measured to exclude individuals in whom infection or inflammation have increased serum ferritin. Both methods probably underestimate iron stores in pregnancy, especially the ferritin method.

Technical Abstract:
Background: To develop global programs for the control of iron deficiency, simple, low-cost, accurate indicators of iron status are needed.
Objectives: To compare estimates of body iron stores, as calculated from either plasma ferritin concentration alone (BI-ferritin) or the ratio of plasma transferrin receptor/ferritin
(BI-TfR/ferritin).
Methods: Data were analyzed from four previously completed randomized intervention trials that enrolled infants, school children or pregnant women (total N=1189, after excluding subjects with elevated CRP).
Results: The correlation coefficients between BI-ferritin and BI-TfR/ferritin were greater than 0.95 for all studies. The Kappa index ranged from 0.5-1.0. All the sensitivities of BI-ferritin for identifying individuals with low Fe stores (defined as
BI-TfR/ferritin<0 mg/kg BW) were greater than 0.90. All the specificities were greater than 0.90, except for the study of pregnant women (specificity= 0.66). The effect sizes of iron intervention trials were significantly greater for change in iron
reserves estimated by BI-TfR/ferritin than by BI-ferritin for 2 studies with larger effect sizes (1.11 vs 1.00 and 1.56 vs 1.44; p<0.05) and 1 study with medium effect size (0.70 vs 0.57; p<0.05). However, there were no significant differences between
estimates of these effect sizes for 1 study with a medium effect size and 1 study with a smaller effect size (0.83 vs 0.78 and 0.35 vs 0.37; p>0.2). Conclusion: Plasma ferritin concentration alone provides a good approximation of total body Fe reserves, as estimated by the ratio of transferrin receptor/ferritin, based
on high correlation, sensitivity and specificity among non-pregnant individuals with non-elevated CRP.